Normally, computer hard disks are subject to a number of differing analog and digital tests as part of the manufacturing process. Fundamentally, these tests are aimed at two separate, although related goals. The first of these goals is the detection of bad, or inoperable, disk drives that, for whatever reason, do not fall within required manufacturing specifications. The second goal of drive testing is the construction of a drive flaw map. During flaw map construction, each sector within the magnetic media of the disk is repeatedly tested. Sectors that are inoperable are mapped to new sectors allocated from a reserved portion of the disk drive. In some cases, of course, a drive will be found to have too many flaws, and the drive will fall outside of required manufacturing specifications.
Traditionally, testing of disk drives has been performed by devices known as drive testers, a process that has several disadvantages. One such disadvantage is caused, in part, by the rapid growth in disk drive technology which has increased drive capacity and performance at a rapid rate. Unfortunately, the performance and capacity of disk drive testers has not matched the rapid pace of disk drive development. As a result, testing of drives has become an increasingly time consuming part of the manufacturing process. This problem is compounded by the relative expense of disk drive testers which limits the number of testers that may be employed economically on a single manufacturing line.
A second disadvantage associated with traditional disk drive testers is the inability of such devices to provide the type of failure prediction that may be usefully employed in high reliability computing environments. More specifically, it is generally the case that disk drives experience an increasing number of defects over the life-span of the drive. These defects, which arise after the drive has left the manufacturing environment, are known as "grown defects." Unfortunately, the great size of modern disk drives has made it relatively impractical to exhaustively re-test a functioning drive.